1. Field of the Invention
The present invention relates to a power supply for a motor, and more particularly, to a motor power supply including a soft charging circuit, an over voltage protection circuit and a dynamic braking circuit.
2. Description of the Related Art
A three-phase motor has three windings and is driven by three-phase power. To supply the three-phase power, as shown in FIG. 1, a motor power supply for the three-phase motor comprises an AC (alternating current) power supply 101 which supplies commercial AC power (110V/220V), a rectifier 103 which rectifies the commercial AC power from the AC power supply 101 into DC power, a capacitor 115 which smoothes the rectified power from the rectifier 103, and an inverter 116 which converts the DC power from the capacitor 115 into AC power having three phases and a variable frequency.
The inverter 116 includes a PWM (pulse width modulation) part (not shown) to generate a square wave signal for PWM, and a plurality of transistors which are turned on/off in response to the square wave signal of the PWM part. Further, the motor power supply comprises a microprocessor (not shown) which turns on/off the transistors of the inverter 116 in response to the square wave signal of the PWM part and modulates the power frequency to control a rotation speed of the motor 117.
Generally, the motor power supply further comprises a soft charging circuit to protect the capacitor 115 from an excessive inrush current being supplied when power is first supplied, an over voltage protection circuit to protect the capacitor 115 from over voltage, and a dynamic braking circuit to decrease the rotation speed of the three-phase motor 117 more rapidly when the motor is to be stopped or reversed in rotation.
The soft charging circuit is employed for protecting the capacitor 115 from being charged with a heavy inrush current when the power is first supplied, and comprises an inrush current limiting resistor 102 provided between the rectifier 103 and the capacitor 115, and a relay 111 which is turned off to allow the rectified power to be supplied from the rectifier 103 to the capacitor 115 by passing through the inrush current protection resistor 102 or turned on to allow the rectified power to be supplied from the rectifier 103 to the capacitor 115 without passing through the inrush current limiting resistor 102. With this configuration, when the power is first supplied, the relay 111 is turned off, and the inrush current limiting resistor 102 limits the inrush current, thereby protecting the capacitor 115 from damage due to the inrush current when the power is first supplied.
The over voltage protection circuit comprises an over voltage protection resistor 112 and an over voltage protection switching element 114 which are connected with each other in series. The series combination of the switching element 114 and the overvoltage protection resistor 112 is connected in parallel with the capacitor 115, and an over voltage protection diode 113 connected in parallel with the over voltage protection resistor 112. While the motor 117 is driven, in the over voltage protection circuit, the over voltage protection switching element 114 is turned on when a voltage regenerated from the motor 117 through the inverter 116 and applied to opposite ends (P and N) of the capacitor 115 reaches a predetermined over voltage, so that the over voltage protection resistor 112 dissipates the over voltage power into heat energy, thereby protecting the capacitor 115 from damage due to the over voltage.
The dynamic braking circuit comprises a dynamic braking resistor network 120 and a relay 122 which connects resistors RD1, RD2 and RD3 to power input terminals U, V and W, respectively, of the motor 117. The dynamic braking circuit brings the motor 117 to a stop when the motor 117 is not being driven, and prevents the motor 117 from being forcibly rotated by an external force after the motor 117 is stopped. Here, the relay 122 is turned off while the motor 117 is driven, and turned on when the motor 117 is braked or after the motor 117 is stopped, thereby bring the motor 117 to a sudden stop or preventing the motor 117 from being forcibly rotated by the external force.
However, in the conventional motor power supply, the soft charging circuit is operated only when the power is first supplied. In other words, the soft charging circuit is not needed in the state that the power is being supplied and the capacitor 115 is charged. Further, the over voltage protection circuit is needed only when the motor 117 is controlled in the state that the capacitor 115 is sufficiently charged with the power and has a stable voltage. Further, the dynamic braking circuit is operated only when the motor 117 is braked or after the motor 117 is stopped. Like the soft charging circuit, the dynamic braking circuit is not needed while the motor 117 is normally driven in the state that the power is being supplied and the capacitor 115 is charged.
Thus, in the conventional motor power supply, the foregoing circuits are independently provided, so that elements are duplicated, thereby increasing the size of the motor power supply and a production cost thereof.